PACK RAT AS AN ENEMY OF NATURAL REPRODUCTION 421 



Only two things appear to prevent rat damage: the presence of 

 needles, and the breaking of the bark into plates and scales. From 

 previous observations it is known that this rat can make his way over 

 needles, but it is quite probable that in these cases the needles bent 

 outward and away from the stem, affording a means for getting over 

 them. In the case of the reproduction, however, both dead and live 

 needles are more or less reflexed and bent back towards the stem, 

 forming an inverted spine-tipped cone which restrains the rat from 

 further progress in that direction. 



The flaking and breaking of the bark is equally effective. Occa- 

 sionally the bark of very young Jeffrey pines is rough, scaly, and 

 broken, but this feature in the reproduction is not a common one and 

 is found in relatively few scattered trees throughout the timbered 

 belt on the Angeles Forest, as far as I am aware. As the tree grows 

 larger the bark eventually breaks, although trees of the common type 

 do not show this cracking to any extent for from 10 to 20 years, 

 and then this process proceeds upward on the tree at about the same 

 rate as the tree grows in height. This bark is always thicker and 

 heavier than the smooth bark, though to all appearances it is as juicy 

 and as tender inside as the smooth bark. 



From observations covering two years it has been found that the 

 damage occurs in the late summer or fall, from September to Novem- 

 ber, and seems to depend on the character of either the spring or the 

 summer season. In summers when showers are not uncommon the 

 amount of injury is negligible, but, following a dry summer and with 

 an equally dry fall, the rats appear to be more active and to do the 

 greater amount of damage. To prove this definitely, ho>vever, requires 

 still further work. • 



If the theory here advanced is correct, it is relatively simple to 

 account for this activity on the part of these rodents on the ground 

 that they are searching for moisture. Chaparral bark, it is known, is a 

 common source of food, and as the rats have no access to water, it is 

 probable that all the water required for sustenance is supplied by their 

 food. In the late summer, even the rather succulent Cea.nothus bark 

 becomes dry and it is quite natural that the rat should go to the 

 source of the most moisture, which without doubt at this time is the 

 pine bark. It is quite possible that the presence of resin in the pines 

 is a repellent, but in times of a lack of water this is not enough to 

 check the animal. 



